Introducing operator characteristic curves to define appropriate frequency of quality control tests: A case study involving whole breast radiotherapy image guidance

• Published in: Physica medica 2020-01, Vol.69, p.275-280
• Main Authors: Bissonnette, Jean-Pierre, Neath, Catherine, Hart, Margaret
• Format: Article
• Language: English
• Subjects: Image guidance; Operator characteristic curves; QC test frequency; Whole breast radiotherapy
• ISSN: 1120-1797; 1724-191X
• DOI: 10.1016/j.ejmp.2019.12.010

•Operator characteristic curves have yet to be applied in Medical Physics to define appropriate quality control test frequency.•We use this formalism to define appropriate frequency of IGRT for whole breast radiotherapy, noted for its high variation.•Using setup errors measured at a single institution, we compare various IGRT frequencies encountered for whole breast RT.•On average, the likelihood of 3 setups out of tolerance (>5 mm towards the mediastinum) is unlikely (5 mm in the direction of the mediastinum was established. Using operator characteristic curves, we compared the effectiveness of various image-guidance schedules in dealing with such errors. We also calculated the dosimetric impact of undetected errors. Setup errors >5 mm towards the mediastinum for this cohort were unlikely, at 2.7%. Imaging half of the fractions protects most patients against three or more undetected errors. Undetected, such an error increases, on average, the maximum dose to 10 cm3 of the heart by 50 cGy, the mean heart dose by 4 cGy, and the left lung V20Gy by 0.2%; therefore, the clinical impact is minute. Given that detected positional errors outside of tolerance are corrected, their residual likelihood decreases with the ratio of fractions being imaged. For most tangential breast radiotherapy patients, setup errors >5 mm towards the mediastinum are unlikely, and their dosimetric impact is remote. Imaging half of the fractions of a course of whole breast radiotherapy prevents these errors to occur more than twice.